The present invention relates to a cooling apparatus equipped with a refrigerant circuit in which a compressor, a gas cooler, pressure reducing means, an evaporator and the like are connected in an annular shape, and carbon dioxide is sealed as a refrigerant.
In a conventional cooling apparatus of such a kind, e.g., a showcase installed at a store, a refrigerant circuit is constituted by sequentially connecting a compressor, a gas cooler (condenser) and diaphragmming means (capillary tube or the like) which constitute a condensing unit and an evaporator installed on a showcase main body side through a pipe in an annular shape. A refrigerant gas compressed by the compressor to become high in temperature and pressure is discharged to the gas cooler. Heat is radiated from the refrigerant gas at the gas cooler, and then the refrigerant gas is diaphragmmed by the diaphragmming means to be fed to the evaporator. The refrigerant evaporates there, and absorbs heat from its surroundings to exhibit a cooling function, thereby cooling the chamber (space to be cooled) of the showcase (e.g., see Japanese Patent Application Laid-Open No. 11-257830).
Recently, in order to deal with global environmental problems, there has been developed a device which uses carbon dioxide (CO2) as a natural refrigerant without using conventional flon even at a refrigerant cycle of such a kind, and uses a refrigerant cycle for running by setting a high pressure side to supercritical pressure.
In the case of using the carbon dioxide as the refrigerant, however, a compression ratio becomes very high, and a temperature of the compressor itself and a temperature of a refrigerant gas discharged into the refrigerant circuit become high. Consequently, it is difficult to obtain desired cooling efficiency.
Thus, a sealing amount of a refrigerant has been adjusted to be sealed in the refrigerant circuit so that outlet and inlet temperatures of the evaporator of the cooling apparatus can become substantially equal early. That is, in this case, since an amount of a refrigerant sealed in the refrigerant is large, freezing efficiency can be improved. However, under an unstable situation in the refrigerant circuit at the time of starting or the like, an abnormal increase occurs in pressure of the high side, creating a fear of damage to the device.
Especially, in the case of using a capillary tube as pressure reducing means, if the sealing amount of a refrigerant is too large as described above, when pressure of the high side increases, pressure of a low side is also increased to raise an evaporation temperature of the evaporator. Consequently, there is a problem of impossibility of reducing a temperature of the cooled space to a desired low temperature.